1. Field of the Invention
The present invention relates to a liquid crystal element such as a display element or a optical shutter and, to a ferroelectric liquid crystal element and an apparatus using the same. More specifically, the present invention relates to a ferroelectric liquid crystal element and liquid crystal apparatus in which superior display characteristics are attained by improving an orientation state of liquid crystal molecules.
2. Related Background Art
A display element for controlling a transmission beam by a combining a polarization element and the refractive index anisotropy of ferroelectric liquid crystal molecules is proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application No. 56-107216 and corresponding U.S. Pat. No. 4,367,924). The ferroelectric liquid crystal has a chiral smectic C phase (SmC*) or H phase (SmH*) having a non-helical structure. In this state, the ferroelectric liquid crystal assumes a first or second optically stable state in response to an electric field applied, which states are maintained when no electric field is applied. In this manner, the ferroelectric liquid crystal has a bistable property and has a high response speed to a change in electric field. It is expected to be used as a high-speed storage type display element in a variety of applications. In particular, this ferroelectric liquid crystal is expected for an application as a large, high-precision display.
In order to enhance a predetermined drive characteristic of a ferroelectric optical modulation element the liquid crystal sealed between a pair of parallel substrates is faced in a molecular orientation state to cause effective phase transitions between the two stable states.
A transmittance of a liquid crystal element utilizing a birefringence of the liquid crystal through crossed nicols is ##EQU1##
where I.sub.0 is the incident light intensity, I is the transmitted light intensity, .theta. is the tilt angle, .DELTA.n is the refractive index anisotropy, d is the thickness of a liquid crystal layer, and .lambda. is the wavelength of incident light.
The tilt angle .theta. in the non-helical structure appears as an angle of an average molecular axis of liquid crystal molecules having helical arrangements in the first and second orientation states. According to the above equation, maximum transmittance can be obtained when the tilt angle .theta. is given as 22.5.degree.. The tilt angle .theta. in the non-helical structure for realizing the bistable property must be as close as to 22.5.degree..
Methods for orienting ferroelectric liquid crystals should be capable of uniaxially aligning a molecular layer consisting of plural smectic liquid crystal molecules along a normal to the molecular layer, such as by simple rubbing process.
An example of the method of aligning a ferroelectric chiral smectic liquid crystal having a non-helical structure is described in U.S. Pat. No. 4,561,726.
The following problem occurs when the conventional orientation method is used, particularly, an orientation method, using a rubbed polyimide film applied to the bistable ferroelectric liquid crystal having a non-helical structure of Clark and Lagerwall.
According to experiments of the present inventors, it is found that an apparent tilt angle .theta. (i.e., 1/2 an angle formed by two molecular axes of the two stable states) of a non-helical ferroelectric liquid crystal having a aligned by a conventional rubbed polyimide film is less than a tilt angle (i.e., an angle .theta. which is 1/2 a vertex angle of a triangular cone shown in FIG. 4A and described later) of a helical ferroelectric liquid crystal. In particular, the tilt angle .theta. of the non-helical ferroelectric liquid crystal oriented by the conventional rubbed polyimide film generally falls within the range of about 3.degree. to 8.degree., and its transmittance falls within a maximum range of about 3% to 5%.
As can be apparent from the above description, according to Clark and Lagerwall, although the tilt angle of a non-helical ferroelectric liquid crystal must be equal to the tilt angle of a helical ferroelectric, liquid crystal to be bistable, the tilt angle .theta. of the non-helical structure is smaller than the tilt angle .theta. of the helical structure. In addition, it is also found that a cause setting the tilt angle .theta. of the non-helical structure to be smaller than that (.theta.) of the helical structure is based on helical orientation of liquid crystal molecules in the non-helical structure. More specifically, in a non-helical ferroelectric liquid crystal, liquid crystal molecules are continuously twisted from an axis of the liquid crystal molecules adjacent the upper substrate to the axis of the liquid crystal molecules adjacent the lower substrate with respect to the normal to the substrates. For this reason, the tilt angle .theta. of the non-helical structure is smaller than the tilt angle .theta. of the helical structure.
When a rubbed polyimide orientation film is used, it serves as an insulating layer between each electrode and the liquid crystal layer. Therefore when a voltage having one polarity is applied to the chiral smectic liquid crystal to switch the orientation state from the first optically stable state (e.g., a white display state) to the second optically stable state (e.g., a black display state), a reverse electric field V.sub.rev having the other polarity is applied to the ferroelectric liquid crystal after the voltage having one polarity is withdrawn. The electric field V.sub.rev causes an afterimage at the time of display.
This reverse electric field generation phenomenon is described in Akio Yoshida, "SSFLC Switching Characteristics", Lecture Papers on Liquid Crystal Forum, October, 1987, pp. 142-143.
One of the present inventors found the following phenomenon associated with an orientation state of a ferroelectric liquid crystal.
An LP 64 (tradename) available from TORAY INDUSTRIES, INC. or the like having a relatively small pretilt angle is applied to substrates to form orientation films thereon, and the orientation films are rubbed and the substrates are adhered to each other with a gap of about 1.5 .mu.m such that the rubbing direction of one film is the same as that of the other film. A ferroelectric liquid crystal CS1014 (tradename) available from Chisso Kabushiki Kaisha is injected into the space defined between the two substrates of the obtained cell and is sealed. When the temperature of the ferroelectric liquid crystal is decreased, phase transitions shown in FIGS. 2A to 2E are obtained. More specifically, in a state shown in FIG. 2A immediately after the phase transition from a high-temperature phase to an Sc* phase, the ferroelectric liquid crystal takes orientation states (C1 orientation states) 21 and 22 having a small contrast value. When the temperature is further decreased and reaches a given temperature region, zig-zag defects 23 are generated, and orientation states (C2 orientation states) 24 and 25 having large contrast values with respect to these defects appear, as shown in FIG. 2B. When the temperature is further decreased, the C2 orientation state propagates (FIGS. 2C and 2D), and the entire liquid crystal is set in the C2 orientation state (FIG. 2E).
These C1 and C2 orientation states can be described in accordance with a difference in chevron structure of the smectic layer, as shown in FIG. 3. Smectic layers 31 in FIG. 3 are classified into C1 orientation regions 32 and a C2 orientation region 33.
A smectic liquid crystal generally has a layer structure. When the S.sub.A phase transits into an S.sub.C or S.sub.C * phase, the layer, interval is reduced, and the layers have a structure that is bent at the center between the upper and lower substrates (i.e., the chevron structure), as shown in FIG. 3. The bending direction is determined by the C1 and C2 orientation states. However, it is known that the bending direction is determined such that liquid crystal molecules at the substrate boundary form an angle with respect to the substrate surface (pretilt) so that the heads of the liquid crystal molecules are inclined toward the rubbing direction (e.g., the leading ends float). Since the elastic energy of the C1 orientation state is not equal to that of the C2 orientation state, phase transitions occur at the predetermined temperatures described above. In addition, a phase transition may occur due to a mechanical stress.
When the layer structure of FIG. 3 is observed from the top, a boundary 34 from the C1 orientation state to the C2 orientation state in the rubbing direction has a zig-zag pattern and is called a lightning defect, while a boundary 35 from the C2 orientation state to the C1 orientation state in the rubbing direction forms a wide slow curve and is called a hairpin defect. There is provided a conventional liquid crystal element using the C2 orientation state of the C1 and C2 orientation states in favor of high contrast.
A matrix display apparatus using the ferroelectric liquid crystal element described above has perpendicular stripe electrodes on the inner surfaces of a pair of substrates so that the display can be driven by methods disclosed in Japanese Laid-Open Patent Application Nos. 59-193426, 59-193427, 60-156046, and 60-156047.
FIG. 18 shows a drive waveform wherein periods of (1) a "black" erase phase and (2) a selective "white" write phase are assigned to a selection period of one scanning line. In the period of the (1) erase phase, a positive voltage is applied to scanning electrodes to set all or predetermined pixels on the scanning line in the first stable state (referred to as a "black" erase state hereinafter). In the period of the (2) selective "white" write phase, a negative voltage is applied to the scanning electrodes, and a positive voltage is selectively applied to only data electrodes corresponding to pixels (selection pixels) supposed to be inverted to the second stable state (referred to as a "white" state hereinafter). The negative voltage is applied to the data electrodes corresponding to the remaining pixels (semi-selection pixels). An inversion electric field having a threshold value or more is generated by the selection pixel, while an inversion electric field having a value smaller than the threshold value is generated by the semi-selection pixel. "White" is written in the selection pixel, while, a "black" state is maintained in the semi-selection pixel. At this time, the absolute value of the erase phase voltage is always equal to that of the write phase voltage.
In a method of rubbing a polymer film of, e.g., polyimide or polyvinyl alcohol having a relatively small pretilt angle, as the most popular method of controlling orientation of a ferroelectric liquid crystal, the two orientation states (bistable, i.e., splay orientation) wherein the apparent tilt angle .theta. is as small as about 1/2 the true tilt angle .theta. are very important, as previously described.
When multiplex driving is applied to an element having these orientation states by using the drive waveform shown in FIG. 18, sufficiently high contrast cannot be obtained while a sufficient drive margin can be obtained. Therefore, much room for improvement is left.